Elastic collision of atoms with perpendicular final motion

[Dave218]

An atom of mass m1 = m moving in the x direction with speed v1 = v collides elastically with an atom of mass m2 = 5m at rest. After the collision the first atom moves in the y direction. Find the direction of motion of the second atom. ________ ° counterclockwise from the +x-axis

Find the speeds of both atoms (in terms of v) after the collision.
v'1 = ________ v
v'2 = ________ v

The Attempt at a Solution

(a) arctan(5) = 78.7/2 = -39.4° counterclockwise from the +x-axis
(b) tan(39.4) = .821 v = v'2

I don't know what formulas/equations to use here and am unsure on how to get v'2. I have worked out the correct answers for (a) and the first part of (b).

 

[Rawrr!]

How about using the conservation of momentum to work it out? Split the momentum into individual components, in the x and y directions, and work from there. It also may help you visualize the situation if you draw the scenario out, both initial conditions, and final conditions.